If I recall correctly, in the game Seth is described as a young general?
For some reason I always thought that he isn't much older than the twins, but with anime artstyle it's impossible to tell. Sometimes somebody who looks 30-40 years old are barely in their 20s or even less than that, sometimes it's the other way around.
I know the ages of the FE protags were frequently bizzarely low, but what is the difference in this case?